The present invention relates to a method for coating a component, in particular a component of a gas turbine or an aircraft engine, in which the coatings are applied to the component by kinetic cold gas spraying [“K3 coating” in German]. The present invention also relates to a component coated in this manner.
In many fields of technology it is necessary to provide coatings on components in order to protect the component from the effects of the environment. In particular, in environments with high temperatures or aggressive media, such as gas turbines or aircraft engines, components must be protected with wear-resistant layers, armoring, oxidation protection layers and the like. However, diverse tasks and aspects arise in the production of coatings, because many factors must be taken into account which have a mutual impact on one another. Therefore, the coating method must be suitable for the component or the material from which the component is formed and the material bond must interact in a reliable manner with the operating conditions.
Two aspects requiring great attention in the case of coatings for components of aircraft engines or gas turbines are related to the adhesive strength of the coating on the component and preventing crack propagation from the coating into the component. If adhesion is lacking, the coating may flake off reducing the service life of the component, and if there is crack propagation from the coating into the component, the strength of the component and thus the safety of the aircraft engine or the gas turbine is endangered. Consequently, these aspects require special attention and continuous improvement.
One method that is used to coat components of gas turbines or aircraft engines is the so-called cold gas spraying or kinetic cold gas spraying, also called the “K3 method” in German (or kinetic cold gas compaction). With this method, the coating material is accelerated at a high speed onto the component to be coated in the form of particles so that it can be deposited there. It is called cold gas spraying, because the material to be deposited is not heated to a melting temperature, as is the case with thermal spraying or flame spraying, but is used at lower temperatures. A method and a device for cold gas spraying are described in WO 2010/003396 A1 for example.
German Patent Document No. DE 10 2009 018 685 A1 relates to a method for producing an armoring of a blade tip as well as blades and gas turbines produced in this manner, wherein the armoring may likewise be applied by kinetic cold gas spraying. To prevent crack propagation from the armoring into the coated component, DE 10 2009 018 685 A1 proposes providing a porous layer beneath the armoring in order to stop crack propagation at the pores and thereby prevent crack propagation in the base material. Even though a solution to prevent crack propagation from coatings that are produced by cold gas spraying already exists, there continues to be a need for achieving improvement in the case of coatings that are produced by kinetic cold gas spraying, particularly with respect to improving adhesion strength and preventing crack propagation from the coating into the component.
Therefore, it is the object of the present invention to make available a method for coating a component, in particular a component of a gas turbine or an aircraft engine, in which the adhesion of the coating, which is applied by kinetic cold gas spraying is improved and a possible crack propagation from the coating into the component is prevented or at least slowed down. At the same time, it should be possible to execute the coating method in a simple and reliable manner.
The present invention is characterized in that, in the case of coatings that are produced by kinetic cold gas spraying, an improvement in the adhesion strength and a reduction in crack propagation or crack growth from the coating into the component are able to be achieved if a pretreatment of the surface of the component to be coated is carried out in which the surface of the to-be-coated component is cleaned and compacted by blasting media striking the surface. The pretreatment is correspondingly designated as shot peening and the component surface is strengthened thereby. At the same time, a cleaning is carried out, because any adhering dirt and/or thin oxide layers, which form on metallic components in particular, are eliminated.
According to one embodiment, shot peening may be carried out in particular in two or more stages, wherein during the different stages the blasting velocity of the blasting media is varied. A corresponding change in the blasting velocity may also be carried out continuously.
The change in the blasting velocity may be carried out with increasing treatment duration in such a manner that the blasting velocity of the blasting media is increased, i.e., the blasting velocity during the first stage is lower than in the second stage or, in the case of a continuous change in the blasting velocity, is higher at the end of shot peening than at the beginning of shot peening.
Because shot peening is merely supposed to cause a compaction of the surface and/or cleaning of the surface, and embedding of the blasting media used for shot peening is not supposed to occur, increasing the blasting velocity of the blasting media with the treatment duration makes it possible to effectuate a high strengthening of the surface area of the component to be coated without the blasting media getting embedded in the surface.
During shot peening, the blasting velocity of the blasting media may always be kept low enough that no substantial adherence of the blasting media to the surface of the component to be coated occurs during the shot peening. In particular, the blasting velocity of the blasting media may be kept below the speed of sound at the beginning of shot peening, while the blasting velocity may be set above the speed of sound at the end of shot peening.
The strengthened layer formed by the shot peening prevents cracks which have formed in the coating from being able to easily propagate into the component, and for them to be stopped at the interface to the component. The treatment of the surface with the blasting media also causes troublesome oxide layers to be removed so that the adhesive strength of the coating is also increased.
Prior to shot peening, a blast cleaning may also be carried out to clean the surface to be coated, wherein in this case the blasting velocity of the blasting media may be set so low that essentially the surface area of the component to be coated is not strengthened, and only cleaning takes place.
All steps of the method according to the invention may be carried out using one and the same device. Therefore, it is possible to use a device for kinetic cold gas spraying for both the blast cleaning as well as the shot peening and the deposition of the coating itself. In this case, only the blasting media must be changed, because inert particles are used for blast cleaning and/or shot peening, while the coating material is used as the blasting media during deposition of the coating.
Brittle and inert materials, such as ceramic substances, sand, glass beads, in particular tungsten carbide particles or the like, may be used for the blast cleaning and/or shot peening. It is also possible to use ice beads.
A correspondingly coated component is characterized in that there is a compaction or strengthened layer beneath the coating applied by kinetic cold gas spraying in which residual compressive stresses have been introduced, which prevent or reduce crack growth or crack propagation. The strengthened layer in this connection is characterized in that this layer is made up predominantly of the base material of the component to be coated, because the compaction does not take place during the deposition of the coating, but already beforehand. In particular, the strengthened layer may be at least partially, in particular however predominantly substantially free of coating material, in particular on the side of the strengthened layer directed towards the inside of the component.
The enclosed drawings depict the following in a purely schematic manner.